check if function is a generator

Question

I played with generators in Nodejs v0.11.2 and I\'m wondering how I can check that argument to my function is generator function.

I found this way typeof f ===

Answer 1

TJ Holowaychuk's co library has the best function for checking whether something is a generator function. Here is the source code:

function isGeneratorFunction(obj) {
   var constructor = obj.constructor;
   if (!constructor) return false;
   if ('GeneratorFunction' === constructor.name || 'GeneratorFunction' === constructor.displayName) return true;
   return isGenerator(constructor.prototype);
}

Reference: https://github.com/tj/co/blob/717b043371ba057cb7a4a2a4e47120d598116ed7/index.js#L221

Answer 2

Mozilla javascript documentation describes Function.prototype.isGenerator method MDN API. Nodejs does not seem to implement it. However if you are willing to limit your code to defining generators with function* only (no returning iterable objects) you can augment it by adding it yourself with a forward compatibility check:

if (typeof Function.prototype.isGenerator == 'undefined') {
    Function.prototype.isGenerator = function() {
        return /^function\s*\*/.test(this.toString());
    }
}

Answer 3

I checked how koa does it and they use this library: https://github.com/ljharb/is-generator-function.

You can use it like this

const isGeneratorFunction = require('is-generator-function');
if(isGeneratorFunction(f)) {
    ...
}

Answer 4

In the latest version of nodejs (I verified with v0.11.12) you can check if the constructor name is equal to GeneratorFunction. I don't know what version this came out in but it works.

function isGenerator(fn) {
    return fn.constructor.name === 'GeneratorFunction';
}

Answer 5

I'm using this:

var sampleGenerator = function*() {};

function isGenerator(arg) {
    return arg.constructor === sampleGenerator.constructor;
}
exports.isGenerator = isGenerator;

function isGeneratorIterator(arg) {
    return arg.constructor === sampleGenerator.prototype.constructor;
}
exports.isGeneratorIterator = isGeneratorIterator;

Answer 6

function isGenerator(target) {
  return target[Symbol.toStringTag] === 'GeneratorFunction';
}

or

function isGenerator(target) {
  return Object.prototype.toString.call(target) === '[object GeneratorFunction]';
}